This graphic features an artist's impression of a star found in the closest orbit known around a black hole, as reported in our latest press release. This discovery was made using data from NASA's Chandra X-ray Observatory
(shown in the inset where low, medium, and high-energy X-rays are
colored red, green, and blue respectively), plus NASA's NuSTAR telescope
and the Australia Telescope Compact Array.
Astronomers found this extraordinarily close stellar pairing in the
globular cluster named 47 Tucanae, a dense collection of stars located
on the outskirts of the Milky Way galaxy, about 14,800 light years from Earth.
This particular source, known as X9, has been of interest to
scientists for many years. Until a couple of years ago, astronomers
thought X9 contained a white dwarf pulling material from a companion star like the Sun. (Astronomers call a pair of objects orbiting one another a 'binary' system.) However, a team of scientists in 2015 used radio data
to show that X9 likely consisted instead of a black hole pulling gas
from a white dwarf companion. These researchers predicted that the white
dwarf would take only about 25 minutes to orbit the black hole.
New Chandra data likely verify this hypothesis and reveal that the
X-rays change periodically over about 28 minutes. Additionally, Chandra
data show evidence for large amounts of oxygen
in the system, a characteristic for the presence of a white dwarf.
Therefore, a strong case can be made that that the companion star is a
white dwarf, which would then be orbiting the black hole at only about
2.5 times the separation between the Earth and the Moon.
As seen in the artist's illustration, the white dwarf is so close to
the black hole that much of its material is being pulled away. If it
continues to lose mass, this white dwarf may evolve into some exotic
sort of planet or completely evaporate.
In order to make such a close pairing, one possibility is that the
black hole smashed into a red giant star, and then gas from the outer
regions of the star was ejected from the binary. The remaining core of
the red giant would form into a white dwarf, which becomes a binary
companion to the black hole. The orbit of the binary would then have
shrunk as gravitational waves were emitted, until the black hole started
pulling material from the white dwarf. The gravitational waves
currently being produced by X9 have a frequency that is too low to be
detected with Laser Interferometer Gravitational-Wave Observatory
(LIGO). It could potentially be detected with future gravitational wave
observatories in space.
An alternative explanation for the observations is that the binary contains a neutron star,
rather than a black hole, that is spinning faster as it pulls material
from a white dwarf companion via a disk. This process can lead to the
neutron star spinning around its axis thousands of times every second. A
few such objects, called transitional millisecond pulsars,
have been observed near the end of this spinning up phase. The authors
do not favor this possibility as transitional millisecond pulsars have
properties not seen in X9, such as extreme variability at X-ray and
radio wavelengths. However, they cannot disprove this explanation.
In addition to Chandra, NASA's NuSTAR telescope, which observes
higher-energy X-rays, and the radio telescope Australia Telescope
Compact Array were used to make this discovery.
A paper describing these results was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available online. The authors on the paper are Arash Bahramian (University of Alberta), Craig Heinke (Alberta), Vlad Tudor (Curtin University and ICRAR), James Miller-Jones (ICRAR), Slavko Bogdanov (Columbia University), Thomas Maccarone (Texas Tech University), Christian Knigge (University of Southampton), Gregory Sivakoff (Alberta), Laura Chomiuk (Michigan State University), Jay Strader (Michigan State), Javier Garcia (Harvard-Smithsonian Center for Astrophysics), and Timothy Kallman (Goddard Space Flight Center).
NASA's Marshall Space Flight Center in Huntsville, Alabama, manages
the Chandra program for NASA's Science Mission Directorate in
Washington. The Smithsonian Astrophysical Observatory in Cambridge,
Massachusetts, controls Chandra's science and flight operations.
A Quick Look at X9 in 47 Tucanae
Fast Facts for 47 Tucanae:
Scale: Inset image is 1.5 arcmin across (about 6.5 light years)
Category: Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 00h 24m 05s | Dec -72° 04´ 53"
Constellation: Tucana
Observation Date: 27 pointings between March 16, 2000 and January 8, 2006
Observation Time: 359 hours
Obs. ID: 953, 955, 2735-2738, 16527, 15747, 16529, 17420, 15748, 16528, 5542-5546, 6230-6233, 6235-6240
Instrument: ACIS
References: Bahramian, A. et al. 2017, MNRAS [in press]; arXiv:1702.02167
Color Code: X-ray (Red, Green, Blue)
Distance Estimate: About 14,800 light years
Source: NASA’s Chandra X-ray Observatory
